Positron emission tomography is a 3-dimensional imaging technique of nuclear medicine that creates a spatial (3D) map of functional activities in a living organism, including those of human patients. This technique is based on the introduction into a studied organism a positron-emitting radionuclide attached to a biological agent (such as, for example, fluorodeoxyglucose) and the subsequent study of its distribution throughout the studied organism (after a certain time needed for this agent to penetrate the body and to concentrate in areas of interest).
A positron emitted by the radionuclide travels a few millimeters within the biological tissue of the studied organism before annihilation with an electron. This annihilation event generates two 511 keV gamma quanta propagating in almost opposite directions (at 180±0.23°) from each other. These two high-energy photons can be captured in order to determine the approximate position of the annihilation event. In a typical PET system, these photons are absorbed by scintillation elements placed around the studied organism in a ring configuration, generating bursts of lower-energy visible photons, which, in their turn, are registered by photo-detectors attached to each scintillation element. Because each pair of high-energy photons travels in a virtually straight line (commonly referred to as the line of response (LOR)), it is possible to localise the tagged biological agent by calculating LOR intersections (so-called ‘sweet spots’). High-energy photons not arriving in pairs are rejected by using coincidence detection.
In conventional systems, the spatial coordinates of a gamma photon absorption event can be determined only to the precision of one scintillation element size. Because of a greater importance of the transverse coordinates, these scintillation elements have an elongated form with relatively small cross-section facing the centre of the field of view (see, e.g., FIG. 1—Prior Art), which leads to good spatial resolution in the central part of the studied volume, but also to progressive degradation of it towards the periphery caused by attribution of incorrect LOR's to absorption events. It is therefore desirable to determine the positions of photon interactions along the long dimension of the scintillation elements, usually called the depth-of-interaction (DOI). PET scanners providing DOI measurement can assign LORs to photon absorption events more accurately, thus resulting in better resolution uniformity across the field of view.
A number of methods have been so far proposed for addition of the DOI capability, however, most of them rely on additional detector electronics, thereby requiring considerably more complicated hardware and introducing numerous other issues (for example, gamma quant absorption in the front-facing electronics and increased dead space in the detector ring). Among these prior art methods include U.S. Patent Pub. No. 2003/0105397 to Tumer et al., which describes a dual-detector readout where two detectors are disposed at opposite ends of a scintillation crystal bar. Dual photo-detector (PD) readout scintillation blocks cannot be packed as tightly as single PD read-outs resulting in dead spaces between blocks where gamma-quanta cannot be captured. Besides, this scheme requires twice as many PD and other electronic components (as compared to single PD read-outs) thereby increasing production costs.
Phoswitch detectors with two or more different scintillator layers discriminated by their decay time are also known for DOI measurements. For example, U.S. Pat. No. 4,843,245 to Lecomte teaches using multi-layer detector consisting of scintillators with different decay time. Although this approach allows rough DOI indexing, which improves the resolution of PET imaging, it is inherently limited by the discrimination abilities of the detection electronics and by the choice of available scintillation materials. Besides, it requires much longer coincidence windows, leading to more sparse number of events collected and to more false coincidence counts, which all make image reconstruction more difficult.
Another approach disclosed in U.S. Pat. No. 7,091,490 to Sumiya et al., consists of using a multilayer scintillator detector incorporating both light sharing and decay time discrimination. This design requires multiple scintillation crystals with different surface treatment and separated by different transparent, reflective, or opaque optical interfaces (incorporated into the detector element), which arguably makes this approach even more complicated than those based on dual detectors because of the very complex composition of the scintillation element.
One specific method uses light sharing between two adjacent scintillating crystals along the long dimension that does not lead to complication of the detector electronics and allows for the extraction of DOI information on the basis of the ratio of light signals collected by detectors attached to each crystal of the pair. It is the subject of U.S. Pat. No. 7,956,331 to Lewellen et al. and provides the means to add the DOI capability to a PET detector without introduction of any additional electronics, relying on shaped opaque screens and/or special optical interface layers between adjacent crystals and on solid-state micro-pixel detectors.
This approach fulfils the technical requirements for a DOI-capable PET system. However, a number of issues associated with detector manufacturing remain to be solved. For example, like in all conventional PET detectors, the approach outlined in U.S. Pat. No. 7,956,331 to Lewellen et al. requires that each scintillation crystal pair be assembled from individual elements and the opaque and transparent layers for light sharing be interposed between them. In turn, each pair of crystals has to be (mostly manually) assembled into a detector block with appropriate reflection and light isolation layers between them. It is desirable, however, to manufacture scintillation detector blocks in a more industrialised and automated way while keeping the valuable DOI capability. The present invention provides, among other things, an alternative approach to adding DOI capability to PET scintillation detectors in a very cost-effective manner that opens further possibilities of automation and improvement of PET detector manufacture.